MEMS device having a movable electrode

ABSTRACT

A microelectromechanical system (MEMS) device includes a semiconductor substrate, a MEMS including a fixed electrode and a movable electrode formed on the semiconductor substrate through an insulating layer, and a well formed in the semiconductor substrate below the fixed electrode. The well is one of an n-type well and a p-type well. The p-type well applies a positive voltage to the fixed electrode while the n-type well applies a negative voltage to the fixed electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 12/710,773 filedFeb. 23, 2010, which is a continuation application of U.S. Ser. No.11/876,107 filed Oct. 22, 2007, now U.S. Pat. No. 7,696,587 issued Apr.13, 2010 which claims priority to Japanese Patent Application Nos.2006-289063 filed Oct. 24, 2006 and 2007-184020 filed Jul. 13, 2007, allof which are hereby expressly incorporated by reference herein in theirentirety.

BACKGROUND

1. Technical Field

The present invention relates to a Micro Electro Mechanical System(MEMS) device.

2. Related Art

MEMS devices manufactured by using MEMS technology have recently beendrawing great attention. Such MEMS devices include a minute MEMS formedon a semiconductor substrate so as to be utilizable for sensors orresonators. The MEMS is provided with a fixed electrode and a movableelectrode. By bending the movable electrode, an electrostaticcapacitance generated at the fixed electrode is detected to therebyprovide MEMS characteristics.

In general, it has been known that parasitic capacitance included insome circuit wirings such as ICs will adversely affect the electricalcharacteristic of ICs and the like. Parasitic capacitance also occurs inMEMS devices. An adverse effect on the electrical characteristic causedby the parasitic capacitance is worsened as the space between electrodesin the MEMS becomes narrower and the applied frequency becomes higher.

Parasitic capacitance is easily formed between the semiconductorsubstrate and the MEMS when the MEMS is produced through a surface MEMSprocess in which the MEMS is directly formed on an extremely thin oxidefilm or nitride film on a semiconductor substrate This is true even whenthe MEMS occupies a small area.

In particular, an electrostatic type MEMS device for detecting volumedisplacement generated by mechanical displacement of a movable electrodehas an extremely weak output signal. In addition, since an absolutevalue of the volume displacement is not sufficiently large with respectto parasitic capacitance, the signal is easily affected by the parasiticcapacitance.

Further, when the parasitic capacitance is large and the resistance of asurface of the substrate is small, or when a capacitance between thesubstrate and an electrode is large, the signal is easily leaked frompathways other than the original pathway through carriers excited on thesurface of the substrate.

For example, FIG. 16 illustrates a known MEMS device that includes aMEMS formed on an oxide film 111 and a nitride film 112 on asemiconductor substrate 110. The MEMS device is provided with fixedelectrodes and a movable electrode. The fixed electrodes include aninput electrode 113, an output electrode 114 and a driving electrode115. The movable electrode includes a movable portion 116 coupled to theinput electrode 113.

In a MEMS device having the above structure, a high-frequency signal mayleak from the input electrode 113 to the output electrode 114 throughthe surface of the semiconductor substrate 110.

To solve this problem, JP-A-2006-174174 (page 5, lines 7 to 11)discloses a leakage amount reduction technique for a high-frequencysignal to a substrate by collectively and commonly coupling lowerelectrodes of a resonator element (MEMS) so as to reduce an areaoccupied by the wiring of the high-frequency signal.

However, although reducing the area occupied by the MEMS as describedabove is an effective method for decreasing parasitic capacitance,reducing the occupied area is not always easily accomplished due torestrictions of designs and/or production. Therefore, when the areaoccupied by the MEMS is not capable of being successfully reduced, theparasitic capacitance causes an adverse effect to the characteristics ofthe MEMS device.

SUMMARY

A MEMS device is provided to reduce parasitic capacitance between a MEMSand a semiconductor substrate.

A MEMS device according to a first aspect includes: a semiconductorsubstrate; a MEMS including a fixed electrode and a movable electrodeformed on the semiconductor substrate through an insulating layer; and awell formed in the semiconductor substrate below the fixed electrode.The well is one of an n-type well and a p-type well. The p-type well isformed to apply a positive voltage to the fixed electrode while then-type well is formed to apply a negative voltage to the fixedelectrode.

According to this structure, the well is formed in the semiconductorsubstrate below the fixed electrode of the MEMS. A p-type well applies apositive voltage to the fixed electrode of the MEMS. An n-type wellapplies a negative voltage to the fixed electrode of the MEMS.

By forming the well, a surface of the semiconductor substrate providedwith the well becomes depleted. As such, an apparent distance betweenthe electrodes facing each other is increased due to the depletionlayer. As a result, parasitic capacitance in this portion is decreased.Therefore, the parasitic capacitance between the MEMS and thesemiconductor substrate can be reduced, so that leakage of a highfrequency signal through the surface of the semiconductor substrate isprevented, thereby stabilizing the characteristics of the MEMS device.

Further, a voltage may be applied to the well so that the well isdepleted.

According to this structure, the voltage is applied to the well formedin the semiconductor substrate located below the fixed electrode so thatthe well is in a depleted state.

When a voltage having a large absolute value is applied to the fixedelectrode, an inversion layer is generated on the surface of thesemiconductor substrate provided with the well, thereby excitingelectrons. In this state, signal leakage occurs easily on the surface ofthe semiconductor substrate regardless of a depletion capacitance.Therefore, by applying a voltage obtained by subtracting a voltage whenthe well is depleted from a voltage applied to the fixed electrode, thewell can maintain the depletion state, thereby preventing the electronsfrom being excited by the inversion layer generated on the surface ofthe semiconductor substrate provided with the well. Since the well canmaintain the depletion state, the parasitic capacitance between the MEMSand the semiconductor substrate can be reduced. Therefore, leakage ofthe high frequency signal through the surface of the semiconductorsubstrate is prevented, thereby stabilizing the characteristics of theMEMS device.

Further, the MEMS device may satisfy Vp<0, Vwell≧0, and0<|Vp−Vwell|<|Vth|, where Vp is a bias voltage of the MEMS, Vwell is avoltage applied to the well below the MEMS, Vth is a threshold voltageat which an inversion layer is formed in the well when the semiconductorsubstrate is a p-type substrate and the well is an n-type well.

By satisfying the above conditions, when the semiconductor substrate isa p-type substrate and the well is an n-type well, the well formed inthe semiconductor substrate below the fixed electrode is depleted. Then,due to the depletion layer generated in the well, the apparent distancebetween the electrodes facing each other is increased, therebydecreasing the parasitic capacitance in this portion. Therefore, theparasitic capacitance between the MEMS and the semiconductor substratecan be reduced, so that leakage of the high frequency signal through thesurface of the semiconductor substrate is prevented, thereby stabilizingthe characteristics of the MEMS device.

Further, the MEMS device may satisfy Vp>0, Vwell≦0, and0<|Vp−Vwell|<|Vth|, where Vp is the bias voltage of the MEMS, Vwell isthe voltage applied to the well below the MEMS, Vth is the thresholdvoltage at which an inversion layer is formed in the well when thesemiconductor substrate is an n-type substrate and the well is a p-typewell.

By satisfying the above conditions, when the semiconductor substrate isan n-type substrate and the well is a p-type well, the well formed inthe semiconductor substrate below the fixed electrode is depleted. Then,due to the depletion layer generated in the well, the apparent distancebetween the electrodes facing each other is increased, therebydecreasing the parasitic capacitance in this portion. Therefore, theparasitic capacitance between the MEMS and the semiconductor substratecan be reduced, so that leakage of the high frequency signal through thesurface of the semiconductor substrate is prevented, thereby stabilizingthe characteristics of the MEMS device.

A MEMS device according to a second aspect includes: a semiconductorsubstrate; a MEMS including a fixed electrode and a movable electrodeformed on the semiconductor substrate through an insulating layer; and awell formed in the semiconductor substrate below the fixed electrode,the well having the same polarity as a polarity of the semiconductorsubstrate; and an isolation well having a polarity opposite to thepolarity of the well and surrounding the well in the semiconductorsubstrate. The well and the isolation well are in a reverse bias state.Likewise, the isolation well and the semiconductor substrate are in areverse bias state.

According to this structure, the potentials of the semiconductorsubstrate and the well are isolated, thereby enabling operation of theMEMS with a voltage having a high absolute value. As a result, aparasitic capacitance between the MEMS and the semiconductor substrateis reduced. Further, employing such a structure can facilitate the useof the MEMS by integrating it with a circuit such as an IC since thepotential of the well does not affect the potential of the semiconductorsubstrate.

In this case, the MEMS device may satisfy Vp<0, and 0<Vp−Vwell<Vth,where Vp is a bias voltage of the MEMS, Vwell is a voltage applied tothe well below the MEMS, Vth is a threshold voltage at which aninversion layer is formed in the well, when the semiconductor substrateis a p-type substrate, the well is a p-type well, and the isolation wellis an n-type well.

By satisfying the above conditions, when the semiconductor substrate isa p-type substrate, the well is a p-type well, and the isolation well isan n-type well, the well formed in the semiconductor substrate below thefixed electrode is depleted. Then, due to the depletion layer generatedin the well, an apparent distance between the electrodes facing eachother is increased, thereby decreasing the parasitic capacitance in thisportion. Therefore, the parasitic capacitance between the MEMS and thesemiconductor substrate can be reduced, so that leakage of a highfrequency signal through the surface of the semiconductor substrate isprevented, thereby stabilizing the characteristics of the MEMS device.

In this case, the MEMS device may satisfy Vp<0, and 0<Vp−Vwell<Vth,where Vp is the bias voltage of the MEMS, Vwell is the voltage appliedto the well below the MEMS, Vth is the threshold voltage at which aninversion layer is formed in the well, when the semiconductor substrateis an n-type substrate, the well is an n-type well, and the isolationwell is a p-type well.

By satisfying the above conditions, when the semiconductor substrate isan n-type substrate, the well is an n-type well, and the isolation wellis a p-type well, the well formed in the semiconductor substrate belowthe fixed electrode is depleted. Then, due to the depletion layergenerated in the well, the apparent distance between the electrodesfacing each other is increased, thereby decreasing the parasiticcapacitance in this portion. Therefore, the parasitic capacitancebetween the MEMS and the semiconductor substrate can be reduced, so thatleakage of the high frequency signal through the surface of thesemiconductor substrate is prevented, thereby stabilizing thecharacteristics of the MEMS device.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described with reference to theaccompanying drawings, wherein like numbers reference like elements.

FIGS. 1A and 1B show a structure of a MEMS device according to a firstembodiment. FIG. 1A is a schematic plan view of the MEMS device, whileFIG. 1B is a partial schematic sectional view taken along line A-A ofFIG. 1A.

FIGS. 2A through 2D are partial sectional views schematically showing aprocess for manufacturing the MEMS device in the first embodiment.

FIGS. 3A through 3D are partial sectional views schematically showingthe process for manufacturing the MEMS device in the first embodiment.

FIGS. 4A through 4C are partial sectional views schematically showingthe process for manufacturing the MEMS device in the first embodiment.

FIG. 5 is a partial sectional view schematically showing a structure ofa MEMS device according to a first modification.

FIG. 6 is a graph showing a relation between a difference of Vp andVwell (Vp−Vwell) and a capacitance C between a MEMS and a well in thefirst modification.

FIG. 7 is a partial sectional view schematically showing a MEMS deviceaccording to a second modification.

FIG. 8 is a graph showing a relation between a difference of Vp andVwell (Vp−Vwell) and a capacitance C between a MEMS and a well in thesecond modification.

FIGS. 9A and 9B show a structure of a MEMS device according to a secondembodiment. FIG. 9A is a partial schematic plan view of the MEMS device,while FIG. 1B is a partial schematic sectional view taken along line B-Bof FIG. 9A.

FIGS. 10A through 10D are partial sectional views schematically showinga process for manufacturing the MEMS device in the second embodiment.

FIGS. 11A through 11D are partial sectional views schematically showingthe process for manufacturing the MEMS device in the second embodiment.

FIGS. 12A through 12C are partial sectional views schematically showingthe process for manufacturing the MEMS device in the second embodiment.

FIGS. 13A and 13B show a structure of a MEMS device according to a thirdembodiment. FIG. 13A is a schematic plan view of the MEMS device, whileFIG. 13B is a partial schematic sectional view taken along line C-C ofFIG. 13A.

FIG. 14 is a partial sectional view schematically showing a MEMS deviceaccording to a third modification.

FIG. 15 is a partial sectional view schematically showing a MEMS deviceaccording to a fourth modification.

FIG. 16 is a diagram explaining a state of signal leakage occurring to aconventional MEMS device.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Before embodiments of the invention are explained and in order tofacilitate a better understanding, a principle by which a signal leaksfrom pathways other than an original one due to parasitic capacitancegenerated in a semiconductor substrate will be explained.

The phenomenon described above can be explained by using a modelincluding a capacitor having a metal formed on a semiconductor throughan insulator. Hence, a MOS capacitor using a p-type semiconductor isexemplified. In a MOS capacitor using a p-type semiconductor, it isknown that a characteristic of a capacitance-voltage shows: anaccumulation state when a negative voltage is applied to a gate; adepletion state when a positive voltage is applied to the gate; and aninversion state when a large positive voltage is applied to the gate.

In the accumulation state, a carrier (hole) is generated on a surface ofthe substrate and conductor resistance in the vicinity of the surface ofthe substrate lowers. As such, signal leakage in a lateral directioneasily occurs.

On the other hand, in the depletion state, an apparent distance betweenelectrodes facing each other is increased, thereby decreasing aparasitic capacitance in this portion. Therefore, the carrier is notgenerated in the vicinity of the surface of the substrate which makessignal leakage in the lateral direction less likely to occur. Further,in the inversion state, an inversion layer is generated and a carrierhaving an opposite polarity is excited there. Signal leakage on thesurface of the substrate in the lateral direction thus easily occurs.

In the depletion state, the signal leakage in the lateral direction isnot likely to occur on the surface of the substrate. Further, since theMEMS device drives with a high voltage in general, as the voltage(threshold voltage) value at which the inversion layer is generatedincreases, the likelihood that the signal leakage on the surface of thesubstrate occurs reduces.

Further, in a case where a well is formed in the semiconductorsubstrate, when a voltage is applied to the surface of the substrate,the inversion layer is less likely to occur until a higher voltage isapplied.

The voltage at which the inversion layer is generated in a MOS capacitorcan be represented by a formula deriving a threshold voltage of a MOStransistor. The threshold voltage Vt in a case of using a p-type well isshown as formula 1.

$\begin{matrix}{V_{t} = {{\frac{2{k \cdot T}}{q}{\ln\left( \frac{N_{A}}{n_{i}} \right)}} + {\frac{2}{C_{i}}\sqrt{{ɛ_{0} \cdot ɛ_{s} \cdot k \cdot T \cdot N_{A} \cdot \ln}\frac{N_{A}}{n_{i}}}}}} & {{Formula}\mspace{14mu} 1}\end{matrix}$

where the symbols used indicate as follows:

k: Boltzmann constant

T: Temperature

q: Absolute value of electric charge

N_(A): Acceptor concentration

n_(i): Intrinsic carrier concentration

C_(i): Capacitance of insulating film per unit area

ε_(O): Permittivity in vacuum, and

ε_(S): Relative dielectric constant of insulating film.

According to this formula, a threshold voltage at which inversion startsdepends on the acceptor concentration of a part of the semiconductorsubstrate. The acceptor concentration can be nearly approximated by acarrier concentration of the well. Therefore, it is apparent that thehigher the carrier concentration is, the more it is likely that the MOScapacitor can maintain a depletion state up to a higher voltage.

Further, in a case where a well is not formed and a p-type siliconsubstrate is used as it is, a carrier concentration of the substratebecomes smaller than a case where the well is formed. Therefore,according to formula 1, it is understandable that the voltage at whichthe inversion layer is generated becomes low, resulting in narrowing avoltage range used in the depletion state. Accordingly, forming the wellenables reduction of the parasitic capacitance in the MEMS device in awider voltage range.

Further, when the well is formed, it is possible to form an optimumsubstrate structure below the fixed electrode of the MEMS withoutdepending on a type of substrate (a p-type substrate or an n-typesubstrate) to be used. In addition, suppression of the parasiticcapacitance is allowed regardless of the type of substrate used.

Thus, forming the well on the semiconductor substrate enables raisingthe voltage at which the inversion layer is generated and suppressingthe signal leakage of the surface of the substrate.

In addition, it is known that an n-type semiconductor substrate also hasthe accumulation state, the depletion state, and the inversion stategenerated due to a gate voltage. Therefore, similar to the above, aparasitic capacitance is reduced by using the depletion state, while thecarrier is not generated in the vicinity of the surface of thesubstrate. As such, signal leakage in a lateral direction is less likelyto occur.

Next, detailed characteristics of the device when a MEMS is driven whilehaving the depletion state below the fixed electrode thereof will bedescribed. Here, an effect obtained in a case where the MEMS is appliedto a MEMS resonator is explained as an example.

As described above, when the MEMS resonator is driven while asemiconductor substrate is in a depletion state, a value of a parasiticcapacitance to be formed is reduced. Therefore, signals passing throughthat parasitic capacitance are reduced, resulting in a steep resonancepeak.

Further, it is known that when an oscillation circuit is structured tobe coupled with an active circuit, a parasitic capacitance included inthe MEMS resonator is regarded as a parasitic capacitance equivalentlyincluded in a transistor, thereby reducing a negative resistance thatcan be generated by the transistor. Consequently, when the parasiticcapacitance of the MEMS resonator decreases, a negative resistance valuethat can be generated by the transistor increases compared to ability ofthe transistor, thereby achieving a low-power-consumption circuit.

On the other hand, when a well is not formed, a bias voltage that can beapplied to the MEMS resonator decreases. When a bias voltage of athreshold value or more is applied to the MEMS resonator, the substratelocated below the fixed electrode of the MEMS is in the inversion state.Therefore, electrons as a few carriers are excited on a surface of thesubstrate, so that the signal easily flows in the lateral direction.Further, since a parasitic capacitance between the fixed electrode andthe substrate increases in accordance with the above, the parasiticcapacitance of the MEMS resonator increases equivalently. As a result,adverse effects such as that a resonance peak of the resonator losessteepness (degradation of Q value) arise.

It has been explained above that electric characteristics of the MEMScan be improved by using the substrate below the fixed electrode of theMEMS in a depletion state.

Hereinafter, embodiments of the invention will be described withreference to the accompanying drawings.

First Embodiment

FIGS. 1A and 1B show a structure of a MEMS device according to a firstembodiment. FIG. 1A is a schematic plan view of the MEMS device, whileFIG. 1B is a partial schematic sectional view taken along line A-A ofFIG. 1A.

A MEMS device 1 is provided with a MEMS 30, a wiring layer 27, and apassivation film 28 on a semiconductor substrate 10. The wiring layer 27is formed to surround the MEMS 30, while the passivation film 28 extendsfrom a top of the wiring layer 27 to above the MEMS 30, and includes anopening 29 formed therein.

The semiconductor substrate 10 is a p-type substrate made of silicon. Onthe semiconductor substrate 10, a silicon oxide film 11 is formed, andfurther a silicon nitride film 12 is formed on the silicon oxide film11. Then, on the silicon nitride film 12, the MEMS 30 is provided. TheMEMS 30 is made of polysilicon, and includes a fixed electrode 20 and amovable electrode 26. The fixed electrode 20 is disposed on the siliconnitride film 12, and provided with input electrodes 21 a and 21 b, andan output electrode 22. The movable electrode 26 is supported at bothsides by portions rising from the input electrodes 21 a and 21 b so asto be held in the air.

An end of the input electrode 21 a extends to the wiring layer 27surrounding the MEMS, and is coupled to a wiring 31. The wiring layer 27is made by laminating an insulating film such as a SiO₂ film. The wiring31 going through the wiring 27 is coupled to an aluminum wiring 32 froma coupling pad formed on the wiring 31.

An end of the output electrode 22 extends to the wiring layer 27surrounding the MEMS, and is coupled to a wiring 33, and further to analuminum wiring 34 from a coupling pad formed on the wiring layer 27.

Under the wiring layer 27, an oxide film 24 such as a SiO₂ film isformed to be used as a sacrifice layer for when the MEMS is released byetching.

Further, the semiconductor substrate 10 below the input electrodes 21 aand 21 b, and the output electrode 22, which are the fixed electrodes ofthe MEMS 30 includes a p-type well 13 formed therein. The well 13 isformed in a region including the MEMS 30 in a plan view.

Further, the passivation film 28 is formed so as to extend from on thewiring layer 27 to above of the MEMS 30. The passivation film 28includes the opening 29 formed therein. The MEMS 30 is released byetching the wiring layer 27 and the oxide film 24 from the opening 29,forming a cavity 35 to dispose the MEMS 30 between the passivation film28 and the semiconductor substrate 10. Note that a fixed voltage isapplied to the well 13.

In the MEMS device 1 having such a structure, when a direct-currentvoltage is applied to the movable electrode 26 through the inputelectrode 21 a of the MEMS 30, a potential difference is generatedbetween the movable electrode 26 and the output electrode 22, resultingin an electrostatic force acting between the movable electrode 26 andthe output electrode 22. Here, when an alternating-current voltage isfurther applied to the movable electrode 26, the electrostatic forcevaries such as being bigger or smaller. Then, the movable electrode 26oscillates to be closer or further from the output electrode 22. At thistime, since transfer of charge occurs on a surface of the outputelectrode 22, an electric current flows into the output electrode 22.Then, the oscillation is repeated. As such, a specific resonancefrequency signal is output from the output electrode 22. When thevoltage applied to the MEMS 30 is equal to or less than an inversionvoltage of the well, the well 13 should be grounded.

On the other hand, when the voltage applied to the MEMS 30 is equal toor more than the inversion voltage of the well described above, avoltage in which a depletion state can be maintained is applied to thewell 13.

For example, when a driving voltage of the MEMS is 8 V and a potentialin which an inversion layer is generated in the well 13 is 7 V, apotential difference between the well 13 and the MEMS 30 is 5 V byapplying a voltage of 3 V to the well 13. Here, the well 13 of thesemiconductor substrate 10 maintains a depletion state withoutgenerating the inversion layer. In this case, in the vicinity of thewell 13, a well (n-type well) having an opposite polarity is arranged(not shown) as a guard ring, and used by applying a voltage whoseabsolute value is equal to or more than the voltage value applied to thewell 13 and has the same polarity as the well 13. For example, when avoltage of 3V is applied to the well 13, a voltage of 5V is applied tothe guard ring portion in the vicinity of the well 13 to be used.

Next, a method for manufacturing a MEMS device having the structureabove will be explained.

FIGS. 2A through 4C are partial sectional views schematically showing aprocess for manufacturing the MEMS device. First, as shown in FIG. 2A,the silicon oxide film 11 is formed on the semiconductor substrate 10made of silicon by thermal oxidation. Next, as shown in FIG. 2B, boron(b) ions are implanted into a predetermined region of the semiconductorsubstrate 10 so as to form the well 13 that is a p-type well.Subsequently, as shown in FIG. 2C, the silicon nitride film 12 is formedon the silicon oxide film 11. Then, as shown in FIG. 2D, after apolysilicon film is formed on the silicon nitride film 12, the inputelectrodes 21 a and 21 b, and the output electrode 22, which are thefixed electrode 20 of the MEMS, are formed by patterning.

Next, as shown in FIG. 3A, the oxide film 24 such as a SiO₂ film isformed on the input electrodes 21 a and 21 b, and the output electrode22. Next, as shown in FIG. 3B, an opening hole 25 is formed in the oxidefilm 24 on the input electrodes 21 a and 21 b. Subsequently, apolysilicon film is formed on the oxide film 24, and patterned. Then, asshown in FIG. 3C, the movable electrode 26 of the MEMS is formed byetching. Thereafter, as shown in FIG. 3D, the wiring layer 27 is formedwith wiring (not shown) layered through an insulating film such as aSiO₂ film.

Next, as shown in FIG. 4A, the passivation film 28 is formed on thewiring layer 27. Subsequently, as shown in FIG. 4B, an opening 29 isformed in the passivation film 28 formed above the MEMS.

Then, as shown in FIG. 4C, the wiring layer 27 and the oxide film 24 areetched by an acid etchant applied through the opening 29, therebyreleasing the MEMS 30. At this time, the cavity 35 is formed between thesemiconductor substrate 10 and the passivation film 28. According to theforegoing, the MEMS device 1 shown in FIG. 1 is produced.

Accordingly, the MEMS device 1 of the first embodiment has the well 13formed below the fixed electrode 20 of the MEMS 30, and a positivevoltage is applied to the fixed electrode 20 of the MEMS 30. The well 13is a p-type well. Further, a fixed voltage is applied to the well 13formed in the semiconductor substrate 10 located below the fixedelectrode 20 so that the well 13 is depleted.

Accordingly, forming the well 13 and applying a fixed voltage to thewell 13 so as to be depleted makes the surface of the semiconductor bedepleted. Since an apparent distance between the electrodes facing eachother is increased due to a depletion layer, a parasitic capacitance atthis portion is decreased. Therefore, a parasitic capacitance betweenthe MEMS 30 and the semiconductor substrate 10 is reduced, so thatleakage of a high frequency signal through the surface of thesemiconductor substrate 10 is reduced, thereby stabilizing thecharacteristics of the MEMS device 1.

First Modification

Next, a first modification on the combination of polarities of thesemiconductor substrate and the well in the first embodiment will beexplained. In the first modification, the semiconductor substrate is ap-type substrate, while the well is an n-type well. Further, thesemiconductor substrate includes a circuit element formed thereon, and apotential of the semiconductor substrate is set at a common potential, 0V.

FIG. 5 is a partial sectional view schematically showing a MEMS deviceaccording to the first modification. A MEMS device 5 is provided with aMEMS (here, only an input electrode 131 in the form of a fixed electrodeis shown and a movable electrode is omitted), a wiring layer 127, and apassivation film 128 on a semiconductor substrate 120. The wiring layer127 is formed around the MEMS, while the passivation film 128 is formedon the wiring layer 127.

The semiconductor substrate 120 is a p-type substrate made of silicon.On the semiconductor substrate 120, a silicon oxide film 121 is formed,and further a silicon nitride film 122 is formed on the silicon oxidefilm 121. Then, on the silicon nitride film 122, a MEMS is provided.Since a structure of the MEMS is the same as that of the MEMS explainedin FIG. 1, a detailed description is omitted here.

Further, the semiconductor substrate 120 below the input electrode 131,which is the fixed electrode of the MEMS, includes an n-type well 123formed therein. The well 123 is formed in a region including the MEMS ina plan view.

Further, an electrode 125 is formed in a part of the well 123, andcoupled to an upper surface of the passivation film 128 by a wiring 126through the wiring layer 127.

A positive voltage is applied to the well 123 through the wiring 126. Onthe other hand, a negative voltage is applied to the input electrode 131of the MEMS.

Here, a threshold voltage in which an inversion layer is generated inthe well 123 is Vth, while a bias voltage applied to the MEMS is Vp, anda voltage applied to the well 123 below the MEMS is Vwell.

A relation between a difference between Vp and Vwell (Vp−Vwell) and acapacitance C between the MEMS and the well in the above state is shownby a graph in FIG. 6.

When the semiconductor substrate 120 is a p-type substrate, and the well123 is an n-type well, the threshold voltage Vth is less than 0 (zero).When the voltage of Vp−Vwell is positive, the well is in an accumulationstate. Therefore, a value of the capacitance C between the MEMS and thewell is large which results in a large parasitic capacitance. A range ofVp−Vwell from the voltage of 0 (zero) to a threshold voltage Vth is arange in which the well is depleted. Therefore, the capacitance Cbetween the MEMS and the well becomes small from 0 V toward thethreshold voltage Vth, thereby the parasitic capacitance is also gettingsmall. Further, when the capacitance C is smaller than the thresholdvoltage Vth, the well is in an inversion state. As described above, byusing the well in a depletion state, the parasitic capacitance betweenthe MEMS and the semiconductor substrate is decreased. In addition,signal leakage in a lateral direction in the vicinity of the substrateis less likely to occur.

In order to deplete the well, the conditions Vp<0, Vwell≧0, and0<|Vp−Vwell|<|Vth| should be satisfied.

By satisfying the above conditions, when the semiconductor substrate 120is a p-type substrate and the well 123 is an n-type well, the well 123formed in the semiconductor substrate below the fixed electrode isdepleted. Then, due to a depletion layer generated in the well 123, anapparent distance between the electrodes facing each other is increased,thereby decreasing the parasitic capacitance in this portion. Therefore,the parasitic capacitance between the MEMS and the semiconductorsubstrate 120 can be reduced, so that leakage of a high frequency signalthrough the surface of the semiconductor substrate 120 is prevented,thereby stabilizing the characteristics of the MEMS device 5. Further,employing such a structure can facilitate the use of the MEMS byintegrating it with a circuit such as an IC.

Second Modification

Next, another modification on the combination of polarities of thesemiconductor substrate and the well in the first embodiment will beexplained. In the second modification, the semiconductor substrate is ann-type substrate, and the well is a p-type well. Further, thesemiconductor substrate includes a circuit element formed thereon, and apotential of the semiconductor substrate is set at a common potential, 0V.

FIG. 7 is a partial sectional view schematically showing a MEMS deviceaccording to the second modification. A MEMS device 6 is provided with aMEMS (here, only an input electrode 151 in the form of a fixed electrodeis shown, and a movable electrode is omitted), a wiring layer 147, and apassivation film 148 on a semiconductor substrate 140. The wiring layer147 is formed around the MEMS, while the passivation film 148 is formedon the wiring layer 147.

The semiconductor substrate 140 is an n-type substrate made of silicon.On the semiconductor substrate 140, a silicon oxide film 141 is formed,and further a silicon nitride film 142 is formed on the silicon oxidefilm 141. Then, on the silicon nitride film 142, the MEMS is provided.Since the structure of the MEMS is the same as that of the MEMSexplained in FIG. 1, a detailed description is omitted here.

Further, the semiconductor substrate 140 below the input electrode 151,which is the fixed electrode of the MEMS, includes a p-type well 143formed therein. The well 143 is formed in a region including the MEMS ina plan view.

Further, an electrode 145 is formed in a part of the well 143, andcoupled to an upper surface of the passivation film 148 by a wiring 146through the wiring layer 147.

A negative voltage is applied to the well 143 through the wiring 146. Onthe other hand, a positive voltage is applied to the input electrode 151of the MEMS.

Here, a threshold voltage in which an inversion layer is generated inthe well 143 is Vth, while a bias voltage applied to the MEMS is Vp, anda voltage applied to the well 143 below the MEMS is Vwell.

A relation between a difference between Vp and Vwell (Vp−Vwell) and thecapacitance C between the MEMS and the well in the above state is shownby a graph in FIG. 8.

When the semiconductor substrate 140 is an n-type substrate, and thewell 143 is a p-type well, the threshold voltage Vth is more than 0(zero). When the voltage of Vp−Vwell is negative, the well is in anaccumulation state. Therefore, a value of the capacitance C between theMEMS and the well is large, resulting in a large parasitic capacitance.A range of Vp−Vwell from the voltage of 0 (zero) to the thresholdvoltage Vth is a range in which the well is depleted. Therefore, thecapacitance C between the MEMS and the well becomes small from 0 Vtoward the threshold voltage Vth, thereby the parasitic capacitance isalso getting small. Further, when the capacitance C is larger than thethreshold voltage Vth, the well is in an inversion state. As describedabove, by using the well in a depletion state, the parasitic capacitancebetween the MEMS and the semiconductor substrate is decreased. Inaddition, signal leakage in a lateral direction in the vicinity of thesubstrate is less likely to occur.

In order to deplete the well, the conditions of Vp>0, Vwell≦0, and0<|Vp−Vwell|<|Vth| should to be satisfied.

By satisfying the above conditions, when the semiconductor substrate 140is an n-type substrate, and the well 143 is a p-type well, the well 143formed in the semiconductor substrate 140 below the fixed electrode isdepleted. Then, due to a depletion layer generated in the well 143, anapparent distance between the electrodes facing each other is increased,thereby decreasing the parasitic capacitance in this portion. Therefore,the parasitic capacitance between the MEMS and the semiconductorsubstrate 140 can be reduced, so that leakage of a high frequency signalthrough the surface of the semiconductor substrate 140 is prevented,thereby stabilizing the characteristics of the MEMS device 6. Further,employing such a structure can facilitate the use of the MEMS byintegrating it with a circuit such as an IC.

Second Embodiment

Next, a MEMS device according to a second embodiment will be explained.

In the second embodiment, what differs from the first embodiment is thata well for an input electrode and a well for an output electrode areindividually formed in a semiconductor substrate.

FIGS. 9A and 9B show a structure of the MEMS device according to thesecond embodiment. FIG. 9A is a schematic plan view of the MEMS device,while FIG. 9B is a partial schematic sectional view taken along line B-Bof FIG. 9A. Here, like numerals indicate like elements in the firstembodiment.

A MEMS device 2 is provided with a MEMS 60, a wiring layer 57, and apassivation film 58 on the semiconductor substrate 10. The wiring layer57 is formed to surround the MEMS 60, while the passivation film 58extends from a top of the wiring layer 57 to above the MEMS 60, andincludes an opening 59 formed therein.

The semiconductor substrate 10 is a p-type substrate made of silicon. Onthe semiconductor substrate 10, the silicon oxide film 11 is formed, andfurther the silicon nitride film 12 is formed on the silicon oxide film11. Then, on the silicon nitride film 12, the MEMS 60 is provided. TheMEMS 60 is made of polysilicon, and includes a fixed electrode 50 and amovable electrode 56. The fixed electrode 50 is disposed on the siliconnitride film 12, and provided with input electrodes 51 a and 51 b, andan output electrode 52. The movable electrode 56 is supported at bothsides by portions rising from the input electrodes 51 a and 51 b so asto be held in the air.

An end of the input electrode 51 a extends to the wiring layer 57surrounding the MEMS 60, and is coupled to a wiring 61. The wiring layer57 is made by laminating an insulating film such as a SiO₂ film. Thewiring 61 going through the wiring 57 is coupled to an aluminum wiring62 from a coupling pad formed on the wiring 61.

Further, an end of the output electrode 52 extends to the wiring layer57, and is coupled to a wiring 63. Furthermore, the output electrode 52is coupled to an aluminum wiring 64 from a coupling pad formed on thewiring layer 57.

Under the wiring layer 57, an oxide film 54 such as a SiO₂ film isformed to be used as a sacrifice layer when the MEMS is released byetching.

Further, the semiconductor substrate 10 below the input electrodes 51 aand 51 b, which are the fixed electrode 50 of the MEMS 60, includesp-type wells 43 a and 43 b individually formed therein. Further, thepassivation film 58 is formed so as to extend from on the wiring layer57 to above the MEMS 60. The passivation film 58 includes the opening 59formed therein. The MEMS 60 is released by etching the wiring layer 57and the oxide film 54 from the opening 59, forming a cavity 65 todispose the MEMS 60 between the passivation film 58 and thesemiconductor substrate 10. Note that a fixed voltage is applied to eachof the wells 43 a and 43 b.

In the MEMS device 2 having such a structure, when a direct-currentvoltage is applied to the movable electrode 56 through the inputelectrode 51 a of the MEMS 60, a potential difference occurs between themovable electrode 56 and the output electrode 52, resulting in anelectrostatic force acting between the movable electrode 56 and theoutput electrode 52. Here, when an alternating-current voltage isfurther applied to the movable electrode 56, the electrostatic forcevaries such as being bigger or smaller. Then, the movable electrode 56oscillates to be closer or further from the output electrode 52. At thistime, since transfer of charge occurs on a surface of the outputelectrode 52, an electric current flows into the output electrode 52.Then, the oscillation is repeated, thereby a specific resonancefrequency signal is output from the output electrode 52. When thevoltage applied to the MEMS 60 is equal to or less than an inversionvoltage of the wells, the wells 43 a and 43 b should be grounded.

On the other hand, when the voltage applied to the MEMS 60 is equal toor more than the inversion voltage of the wells described above, avoltage in which a depletion state can be maintained is applied to thewell 43 a and the well 43 b. For example, when a driving voltage of theMEMS 60 is 8 V and a potential in which an inversion layer is generatedin the semiconductor substrate 10 is 7 V, a potential difference betweenthe semiconductor substrate 10 and the MEMS 60 is 5 V by applying avoltage of 3 V to the wells 43 a and 43 b. In this case, the wells 43 aand 43 b of the semiconductor substrate 10 maintain a depletion statewithout generating the inversion layer.

In this case, in the vicinity of the wells 43 a and 43 b, a well havingan opposite polarity is arranged (not shown) as a guard ring, and usedby applying a voltage whose absolute value is equal to or more than avoltage value applied to the well 13 and has the same polarity as thewells 43 a and 43 b. For example, when a voltage of 3 V is applied tothe wells 43 a and 43 b, a voltage of 5 V is applied to a guard ringportion in the vicinity to be used.

Next, a method for manufacturing a MEMS resonator having the structureabove will be explained.

FIGS. 10A through 12C are partial sectional views schematically showinga process for manufacturing the MEMS device.

First, as shown in FIG. 10A, the silicon oxide film 11 is formed on thesemiconductor substrate 10 made of silicon by thermal oxidation. Next,as shown in FIG. 10B, boron (B) ions are implanted into a predeterminedregion of the semiconductor substrate 10 so as to form the wells 43 aand 43 b that are p-type wells. Subsequently, as shown in FIG. 10C, thesilicon nitride film 12 is formed on the silicon oxide film 11. Then, asshown in FIG. 10D, after a polysilicon film is formed on the siliconnitride film 12, the input electrodes 51 a and 51 b, and the outputelectrode 52, which are the fixed electrode 50 of the MEMS, are formedby patterning.

Next, as shown in FIG. 11A, the oxide film 54 such as a SiO₂ film isformed on the input electrodes 51 a and 51 b, and the output electrode52. Afterwards, as shown in FIG. 11B, an opening hole 55 is formed inthe oxide film 54 on the input electrodes 51 a and 51 b. Subsequently, apolysilicon film is formed on the oxide film 54, and patterned. Then, asshown in FIG. 11C, the movable electrode 56 of the MEMS is formed byetching. Further, as shown in FIG. 11D, the wiring layer 57 formed bywiring (not shown) layered through an insulating film such as a SiO₂film.

Next, as shown in FIG. 12A, the passivation film 58 is formed on thewiring layer 57. Subsequently, as shown in FIG. 12B, the opening 59 isformed in the passivation film 58 formed above the MEMS.

Then, as shown in FIG. 12C, the wiring layer 57 and the oxide film 54are etched by coming in contact with an acid etchant through the opening59, thereby releasing the MEMS 60. At this time, the cavity 65 is formedbetween the semiconductor substrate 10 and the passivation film 58.According to the above, the MEMS device 2 as shown in FIG. 9 isproduced.

Accordingly, the MEMS device 2 of the second embodiment includes thewells 43 a and 43 b formed below the fixed electrode 50 of the MEMS 60.To the fixed electrode 50 of the MEMS 60, a positive voltage is applied.The wells 43 a and 43 b are p-type wells. Further, a fixed voltage isapplied to the wells 43 a and 43 b formed in the semiconductor substrate10 located below the fixed electrode 50 so that the wells 43 a and 43 bare depleted.

Accordingly, forming the wells 43 a and 43 b and applying a fixedvoltage to the wells 43 a and 43 b so as to deplete them makes thesurface of the semiconductor be depleted. Since an apparent distancebetween the electrodes facing each other is increased due to a depletionlayer, a parasitic capacitance at this portion is decreased. Therefore,the parasitic capacitance between the MEMS 60 and the semiconductorsubstrate 10 is reduced, so that leakage of a high frequency signalthrough the surface of the semiconductor substrate 10 is reduced,thereby stabilizing the characteristics of the MEMS device 2.

Further, in the second embodiment, since each structure of the substratebelow the input electrode and the output electrode of the MEMS isindividually formed, signal leakage in a lateral direction of thesubstrate can be further reduced. As a result, an insulating propertybetween the input electrodes 51 a and 51 b, and the output electrode 52is further improved, thereby stabilizing the characteristics of the MEMSdevice 2.

Third Embodiment

Next, a MEMS device according to a third embodiment will be explained.

In the third embodiment, what differs from the first embodiment and thesecond embodiment is a structure of a well to be formed in asemiconductor substrate. However, the MEMS is similarly structured tothat of the second embodiment.

FIGS. 13A and 13B show a structure of the MEMS device according to thethird embodiment. FIG. 13A is a schematic plan view of the MEMS device,while FIG. 13B is a partial schematic sectional view taken along a lineC-C of FIG. 13A. Here, like numerals indicate like elements in the firstembodiment. Note that, in these figures, only features that aredistinguishing are schematically shown. The wiring layer surrounding theMEMS and the like described in the embodiments above are thus omitted.

A MEMS device 3 is provided with a MEMS 90 composed of a fixed electrode80 and a movable electrode 86 on the semiconductor substrate 10.

The semiconductor substrate 10 is a p-type substrate made of silicon. Onthe semiconductor substrate 10, the silicon oxide film 11 is formed, andfurther the silicon nitride film 12 is formed on the silicon oxide film11. Then, on the silicon nitride film 12, the MEMS 90 is provided. TheMEMS 90 is made of polysilicon, and includes the fixed electrode 80 andthe movable electrode 86. The fixed electrode 80 is disposed on thesilicon nitride film 12, and provided with an input electrode 81, adriving electrode 82, and an output electrode 83. The movable electrode86 is supported at one side by a portion rising from the input electrode81 so as to be held in the air.

Further, the semiconductor substrate 10 below the input electrode 81,the driving electrode 82, and the output electrode 83, which compose thefixed electrode 80 in the MEMS 90 includes a p-type well 70 having thesame polarity as that of the semiconductor substrate 10 formed therein.Further, an n-type isolation well 71 having a polarity opposite to thatof the well 70 is formed to surround the well 70. The well 70 and theisolation well 71 are formed in a region including the MEMS 90 in a planview.

A fixed voltage Vwp is applied to the well 70, while a fixed voltage Vwnis applied to the isolation well 71 so that a relation between them isVwp<Vwn.

At this time, the voltage applied to the well 70 is a voltage that thewell can maintain a depletion state. For example, when a driving voltageof the MEMS 90 is 10 V and a potential in which an inversion layer isgenerated in the semiconductor substrate 10 is 7 V, a potentialdifference between the well 70 and the MEMS 90 is 5 V by applying avoltage of Vwp=5V to the well 70. In this case, the well 70 of thesemiconductor substrate 10 maintains the depletion state withoutgenerating the inversion layer. Further, a voltage of Vwn=6V is appliedto the isolation well 71, while a voltage to be a reverse bias isapplied to the n-type well or the p-type well adjacent to the isolationwell 71.

Accordingly, the MEMS device 3 of the third embodiment has the well 70that is a p-type well and formed below the fixed electrode 80 of theMEMS 90, and a positive voltage is applied to the fixed electrode 80 ofthe MEMS 90. Further, a fixed voltage is applied to the well 70 locatedbelow the fixed electrode 80 in the semiconductor substrate 10 so thatthe well 70 is depleted.

Since a surface of the well 70 becomes in the depletion state, anapparent distance between the electrodes facing each other is increaseddue to the depletion layer, decreasing a parasitic capacitance in thisportion. Therefore, the parasitic capacitance between the MEMS 90 andthe semiconductor substrate 10 can be reduced, so that leakage of a highfrequency signal through the surface of the semiconductor substrate 10is prevented, thereby stabilizing the characteristics of the MEMS device3.

Further, the isolation well 71 surrounding the well 70 is formed so thatthe voltage applied to the isolation well 71 is higher than the voltageapplied to the well 70. Accordingly, when the movable electrode 86 ofthe MEMS 90 is driven with a higher voltage, a potential in a portionwhere the MEMS 90 is formed is isolated from others without a currentflow from the well 70 to the isolation well 71. Further, employing sucha structure can facilitate providing a device including the MEMS 90integrated with a circuit such as an IC.

Third Modification

Next, a third modification of the third embodiment will be explained. Inthe third embodiment, a semiconductor substrate is a p-type substrate,and a well is a p-type well, while an isolation well is an n-type well.A voltage is not applied to the isolation well. Further, thesemiconductor substrate includes a circuit element formed thereon, and apotential of the semiconductor substrate is set at a common potential, 0V.

FIG. 14 is a partial sectional view schematically showing a MEMS deviceaccording to the third modification. A MEMS device 7 is provided with aMEMS (here, only an input electrode 171 in the form of a fixed electrodeis shown, and a movable electrode is omitted), a wiring layer 167, and apassivation film 168 on a semiconductor substrate 160. The wiring layer167 is formed around the MEMS, while the passivation film 168 is formedon the wiring layer 167.

The semiconductor substrate 160 is a p-type substrate made of silicon.On the semiconductor substrate 160, a silicon oxide film 161 is formed,and further a silicon nitride film 162 is formed on the silicon oxidefilm 161. Then, on the silicon nitride film 162, the MEMS is provided.Since a structure of the MEMS is the same as that of the MEMS explainedin FIG. 1, a detailed description is omitted here.

The semiconductor substrate 160 below the input electrode 171, which isthe fixed electrode of the MEMS, includes a p-type well 163 having thesame polarity as that of the semiconductor substrate 160 and beingformed therein. The well 163 is formed in a region including the MEMS ina plan view. Further, an n-type isolation well 164 having a polarityopposite to that of the well 163 is formed in the semiconductorsubstrate 160 so as to surround the well 163. Furthermore, a positivevoltage is applied to the input electrode 171 of the MEMS.

An electrode 165 is formed in a part of the well 163, and coupled to anupper surface of the passivation film 168 by a wiring 166 through thewiring layer 167. Applying a positive or negative voltage to theelectrode 165 makes the isolation well 164 and the semiconductorsubstrate 160 be in a reverse bias state.

Here, a threshold voltage in which an inversion layer is generated inthe well 163 is Vth, while a bias voltage applied to the MEMS is Vp, anda voltage applied to the well 163 below the MEMS is Vwell.

A relation between a difference between Vp and Vwell (Vp−Vwell) and thecapacitance C between the MEMS and the well in the above state is thesame as the relation shown by the graph in FIG. 8.

Therefore, when the semiconductor substrate 160 is a p-type substrate,and the well 163 is a p-type well, while the isolation well 164 is ann-type well, the threshold voltage Vth is more than 0 (zero).

When the voltage of Vp−Vwell is negative, the well is in an accumulationstate. Therefore, a value of the capacitance C between the MEMS and thewell is large, resulting in a large parasitic capacitance. A range ofVp−Vwell from the voltage of 0 (zero) to the threshold Vth is a range inwhich the well is depleted. Therefore, the capacitance C between theMEMS and the well becomes gradually small from 0 V toward the thresholdVth, thereby the parasitic capacitance is also getting small. Further,when the capacitance C is larger than the threshold voltage Vth, thewell is in an inversion state. As described above, by using the well ina depletion state, the parasitic capacitance between the MEMS and thesemiconductor substrate is decreased. In addition, signal leakage in alateral direction in the vicinity of the substrate is less likely tooccur.

To deplete the well, the conditions of Vp>0, and 0<Vp−Vwell<Vth need tobe satisfied. In this case, Vwell can be either a positive voltage or anegative voltage as long as the value satisfies the above conditions.

By satisfying the above conditions, when the semiconductor substrate 160is a p-type substrate, the well 163 is a p-type well, and the isolationwell 164 is an n-type well, the well 163 formed in the semiconductorsubstrate 160 below the fixed electrode is in the depletion state. Then,due to the depletion layer generated in the well 163, an apparentdistance between the electrodes facing each other is increased, therebydecreasing a parasitic capacitance in this portion. Therefore, theparasitic capacitance between the MEMS and the semiconductor substrate160 can be reduced, so that leakage of a high frequency signal throughthe surface of the semiconductor substrate 160 is prevented, therebystabilizing the characteristics of the MEMS device 7. Further, employingsuch a structure can facilitate the use of the MEMS by integrating itwith a circuit such as an IC since a potential of the well does notaffect a potential of the semiconductor substrate.

Fourth Modification

Next, another modification on the combination of polarities of thesemiconductor substrate and the well in the third embodiment will beexplained. In a fourth modification, the semiconductor substrate is ann-type substrate, the well is an n-type well, and the isolation well isa p-type well. Further, the semiconductor substrate includes a circuitelement formed thereon, and a potential of the semiconductor substrateis set at a common potential, 0 V.

FIG. 15 is a partial sectional view schematically showing a MEMS deviceaccording to the fourth modification. A MEMS device 8 is provided with aMEMS (here, only an input electrode 191 in the form of a fixed electrodeis shown, and a movable electrode is omitted), a wiring layer 187, and apassivation film 188 on a semiconductor substrate 180. The wiring layer187 is formed around the MEMS, while the passivation film 188 is formedon the wiring layer 187.

The semiconductor substrate 180 is an n-type substrate made of silicon.On the semiconductor substrate 180, a silicon oxide film 181 is formed,and further a silicon nitride film 182 is formed on the silicon oxidefilm 181. Then, on the silicon nitride film 182, the MEMS is provided.Since the structure of the MEMS is the same as that of the MEMSexplained in FIG. 1, a detailed description is omitted here.

The semiconductor substrate 180 below the input electrode 191 of theMEMS, includes an n-type well 183 having the same polarity as that ofthe semiconductor substrate 180 and being formed therein. The well 183is formed in a region including the MEMS in a plan view. Further, ap-type isolation well 184 having a polarity opposite to that of the well183 is formed so as to surround the well 183. Furthermore, a negativevoltage is applied to the input electrode 191 of the MEMS.

An electrode 185 is formed in a part of the well 183, and coupled to anupper surface of the passivation film 188 by a wiring 186 through thewiring layer 187. Applying a negative or positive voltage to theelectrode 185 makes the isolation well 184 and the semiconductorsubstrate 180 be in a reverse bias state.

Here, a threshold voltage in which an inversion layer is generated inthe well 183 is Vth, while a bias voltage applied to the MEMS is Vp, anda voltage applied to the well 183 below the MEMS is Vwell.

A relation between a difference between Vp and Vwell (Vp−Vwell) and thecapacitance C between the MEMS and the well in the above state is thesame as the relation shown by the graph in FIG. 6.

Therefore, when the semiconductor substrate 180 is an n-type substrate,the well 183 is an n-type well, and the isolation well 184 is a p-typewell, the threshold voltage Vth is less than 0 (zero).

When the voltage of Vp−Vwell is positive, the well is in an accumulationstate. Therefore, a value of the capacitance C between the MEMS and thewell is large, resulting in a large parasitic capacitance. A range ofVp−Vwell from the voltage of 0 (zero) to a threshold Vth is a range inwhich the well is depleted. Therefore, the capacitance C between theMEMS and the well becomes small from 0 V toward the threshold Vth,thereby the parasitic capacitance is also getting small. Further, whenthe capacitance C is smaller than the threshold voltage Vth, the well isin an inversion state. As described above, by using the well in adepletion state, the parasitic capacitance between the MEMS and thesemiconductor substrate is decreased. In addition, signal leakage in alateral direction in the vicinity of the substrate is less likely tooccur.

To deplete the well, the conditions of Vp<0, and 0<Vp−Vwell<Vth shouldbe satisfied. In this case, Vwell can be either a positive voltage or anegative voltage as long as the value satisfies the above conditions.

By satisfying the above conditions, when the semiconductor substrate 180is an n-type substrate, the well 183 is an n-type well, and theisolation well 184 is a p-type well, the well 183 formed in thesemiconductor substrate 180 below the fixed electrode is in thedepletion state. Then, due to the depletion layer generated in the well183, an apparent distance between the electrodes facing each other isincreased, thereby decreasing a parasitic capacitance in this portion.Therefore, the parasitic capacitance between the MEMS and thesemiconductor substrate 180 can be reduced, so that leakage of a highfrequency signal through the surface of the semiconductor substrate 180is prevented, thereby stabilizing the characteristics of the MEMS device8. Further, employing such a structure can facilitate the use of theMEMS by integrating it with a circuit such as an IC since a potential ofthe well does not affect a potential of the semiconductor substrate.

1. A microelectromechanical system (MEMS) device, comprising: asemiconductor substrate; a p-type well formed on the semiconductorsubstrate; an insulating layer formed on the p-type well; and a MEMSincluding a first fixed electrode supporting a movable electrode and asecond fixed electrode formed in an opposite direction to the movableelectrode, wherein the first fixed electrode and the second fixedelectrode are formed on the insulating layer on the upper side of thep-type well, wherein a positive voltage is applied to at least one ofthe first fixed electrode and the second fixed electrode.
 2. The MEMSdevice according to claim 1, wherein a voltage is applied to the p-typewell so that the p-type well is in a depletion state.
 3. The MEMS deviceaccording to claim 2, wherein the semiconductor substrate is an n-typesubstrate, wherein Vp>0, Vwell≦0, and 0<|Vp−Vwell|<|Vth| are satisfiedwhen Vp is a bias voltage of the MEMS, Vwell is the voltage applied tothe p-type well below the MEMS, and Vth is a threshold voltage at whichan inversion layer is formed in the p-type well.
 4. The MEMS deviceaccording to claim 1, wherein the movable electrode is formedcontinuously with the first fixed electrode.
 5. The MEMS deviceaccording to claim 1, wherein the movable electrode is formedcontinuously with the first fixed electrode.
 6. A microelectromechanicalsystem (MEMS) device, comprising: a semiconductor substrate; a n-typewell formed on the semiconductor substrate; an insulating layer formedon the n-type well; and a MEMS including a first fixed electrodesupporting a movable electrode and a second fixed electrode formed in anopposite direction to the movable electrode, wherein the first fixedelectrode and the second fixed electrode are formed on the insulatinglayer on the upper side of the n-type well, wherein an negative voltageis applied to at least one of the first fixed electrode and the secondfixed electrode.
 7. The MEMS device according to claim 6, wherein avoltage is applied to the n-type well so that the n-type well is in adepletion state.
 8. The MEMS device according to claim 6, wherein thesemiconductor substrate is a p-type substrate, wherein Vp<0, Vwell≧0,and 0<|Vp−Vwell|<|Vth| are satisfied when Vp is a bias voltage of theMEMS, Vwell is the voltage applied to the n-type well below the MEMS,and Vth is a threshold voltage at which an inversion layer is formed inthe n-type well.